This invention relates to an optical time domain testing instrument.
A conventional optical time domain reflectometer (OTDR) comprises a laser diode that is periodically energized to launch optical interrogation pulses into a fiber under test. Return light received from the fiber under test is applied to a photodetector, such as an avalanche photodiode. The photodetector generates a current signal dependent on the intensity of light incident on the photodetector. The current signal is applied to an input amplifier and is converted to a voltage signal across the input impedance of the amplifier. The voltage signal is amplified by the amplifier and the resulting analog voltage signal is sampled at predetermined times relative to the time of energization of the laser diode. The sample values are processed and stored. The stored sample values can be used to provide a display showing intensity of return light received from the optical fiber as a function of distance from the laser diode.
In order to achieve high resolution, it is necessary that the interrogation pulses launched into the fiber under test be very short. Therefore, the input amplifier must have a very high bandwidth in order to preserve short rise time signal information in the current signal provided by the photodetector. This implies that the impedance across which the current signal is converted to voltage must be small, and therefore the amplitude of the resulting voltage signal is small. Consequently, increase in resolution of an OTDR is accompanied by a reduction in the signal-to-noise ratio of the instrument. The return light that is received from the fiber under test is a combination of Rayleigh backscattered light and fresnel reflection light. The intensity of the backscattered light is much less intense than the reflection light, and is generally of more interest than the backscattered light. The intensity of backscattered light is dependent upon the duration of the interrogation pulse, so that at high resolution the intensity of backscattered light is low and the signal-to-noise ratio is accordingly reduced.
When high intensity light is incident on the detector, for example due to fresnel reflection, detector recovery is not instantaneous and therefore the current provided by the detector immediately after receiving such high intensity illumination does not accurately represent the intensity of light incident on the detector at that time. A laser amplifier comprises a laser diode that is biased so that there is a population inversion in its channel, but the channel is not quite in a lasing condition. When light of the wavelength that would be emitted by the laser diode if it were biased into the lasing condition is introduced into the channel of the laser diode, it precipitates electron transitions such that the optical signal that enters the channel is amplified.
A fiber amplifier comprises an optical fiber doped with a selected impurity, such as erbium. If light of a first wavelength is introduced into the fiber, it causes a population inversion that can be triggered by light of a second wavelength, resulting in amplification of the light of the second wavelength.